<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form >
        <input type="text" id="username"/>
        <input type="password" id="pas"/>
        <input type="submit"  value="登录" id="sub"/>
    </form>
</body>
<script>
    var  f1 = document.getElementById("f1");
    var username = document.getElementById("username");
    var  pas = document.getElementById("pas");
    var  sub = document.getElementById("sub");

    username.onblur=function(){
        var val=username.value;
        if(val.length<3){
            alert("用户名过短");
        }
    }
</script>
</html>